Abstract.Thein situdiet of the copepodUndinula vulgariswas examined by comparing contents of fecal pellets produced on natural food with assemblages of available phytoplankton. Samples were collected in continental shelf waters of the northern Gulf of Mexico.U. vulgariswas an indiscrimi‐nant suspension feeder, ingesting a broad size, shape, and taxonomic array of phytoplankters. Contents of fecal pellets generally matched those of available phytoplankton assemblages. The presence within the same fecal pellets of remains of microplankters ranging over two orders of magnitude in longest dimension suggests that feeding is nonselective, and thatU. vulgarismay rapidly switch between several modes of feedin